Drug delivery devices for setting and dispensing a single or multiple doses of a liquid medicament are as such well-known in the art. Generally, such devices have substantially a similar purpose as that of an ordinary syringe.
Drug delivery devices, in particular pen-type injectors have to meet a number of user-specific requirements. For instance, with patient's suffering chronic diseases, such like diabetes, the patient may be physically infirm and may also have impaired vision. Suitable drug delivery devices especially intended for home medication therefore need to be robust in construction and should be easy to use. Furthermore, manipulation and general handling of the device and its components should be intelligible and easy understandable. Moreover, a dose setting as well as a dose dispensing procedure must be easy to operate and has to be unambiguous.
Typically, such devices comprise a housing or a particular cartridge holder, adapted to receive a cartridge at least partially filled with the medicament to be dispensed. The device further comprises a drive mechanism, usually having a displaceable piston rod which is adapted to operably engage with a piston of the cartridge. By means of the drive mechanism and its piston rod, the piston of the cartridge is displaceable in a distal or dispensing direction and may therefore expel a predefined amount of the medicament via a piercing assembly, which is to be releasably coupled with a distal end section of the housing of the drug delivery device.
The medicament to be dispensed by the drug delivery device is typically provided and contained in a multi-dose cartridge. Such cartridges typically comprise a vitreous barrel sealed in distal direction by means of a pierceable seal and being further sealed in proximal direction by the piston. With reusable drug delivery devices an empty cartridge is replaceable by a new one. In contrast to that, drug delivery devices of disposable type are to be entirely discarded when the medicament in the cartridge has been completely dispensed or used-up.
Drug delivery devices such like pen type injectors also provide a dose indicating mechanism which is operable to display the size of a dose actually set. Typically, the housing of such drug delivery devices comprises a dose indicating window in which an information content, in particular a number representing the size of the dose shows up.
Especially for elderly or impaired patients, reading of such dose indication or of does indicating numbers is sometimes difficult. Typically, the consecutive numbers illustrated by a dose indicating mechanism and representing the size of a dose actually set are also indicative of the actual operational status of the drug delivery device. For instance with injection devices such like pen-type injectors, the dose indicating mechanism may comprise a rotatable sleeve featuring a helical scale with consecutive numbers. During setting of a dose, such a dose indicating sleeve typically rotates in a dose incrementing direction, whereby the numbers displayed in a respective window of the device increment.
During a consecutive dose dispensing operation, the dose indicating sleeve typically revolves in an opposite sense of rotation, hence in a dose decrementing direction until it returns in initial configuration. Typically, in such an initial configuration a number like ‘0’ may show up in the window thereby indicating, that the device is ready for setting and dispensing of a subsequent dose.
Many drug delivery devices further offer the possibility to interrupt a dose dispensing procedure. Respective drive mechanisms of such drug delivery devices may be sensitive to distally and externally applied dispensing forces that are to be exerted by the user of the device during the entire dose dispensing process. With many pen-type injectors, the user may simply have to depress an actuation member in distal direction, e.g. with a thumb. An early or premature release of the actuation member, i.e. before the dispensing process terminates, may immediately interrupt the dispensing and hence the injection procedure.
In such a situation the device is still operable to dispense a residual amount of the dose of the medicament previously set during a dose setting procedure. However, and due to the circumstances causing the patient to interrupt a dispensing procedure the patient or user may be simply unaware that dose dispensing has not terminated yet. In such circumstances there may persist a certain risk that the dose previously set is not completely dispensed and injected. Moreover, in a subsequent dose setting procedure, the user may be confused since dialing and setting of a dose may not start from an initial condition, in which a zero dose indicating number, such like ‘0’ shows up in the dose indicating window.